Intracellular injections of various indicator substances will be made into the outer segments of rods of Bufo marinus in order to permit the measurement of the concentration of ionized calcium in the cytoplasm. Injections of the chelating agent EGTA (which has high specificity for Ca+2 binding) will be made into the outer segments of rods that have been enzymatically isolated from the retinas of lower vertebrates. The effect of the EGTA injection upon the light-induced membrane currents will be measured. These experiments will be done to test the hypothesis that a light-induced increase in cytoplasmic (Ca+2) is an intermediate step in the generation of the vertebrate receptor potential.